Transformer construction

ABSTRACT

The invention relates to a transformer construction comprising a plurality of transformer cores configured to share magnetic flux paths and, as a result, at least one of the cores comprises a post and an associated sidewall having an effective cross-sectional area which is less than that of the post. Such a construction may be employed in a power conditioning unit, for example, for a photovoltaic module, which is configured to operate the cores out of phase from each other. Also described is a transformer winding comprising a longitudinal spine having a first turn emanating from a first portion of the spine in a first transverse direction and a second turn emanating from a second portion of the spine in a second transverse direction, wherein the second transverse direction is opposite to the first transverse direction.

CLAIM OF BENEFIT TO PRIOR APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 13/297,253, filed Nov. 15, 2011, now published asU.S. Publication 2012/0081204. U.S. patent application Ser. No.13/297,253 claims priority to an earlier-filed United Kingdom PatentApplication 1118996.6, filed Nov. 3, 2011. U.S. patent application Ser.No. 13/297,253, now published as U.S. Publication 2012/0081204 andUnited Kingdom Patent Application 1118996.6 are incorporated herein byreference.

FIELD OF THE INVENTION

This invention relates to a transformer construction. Particularly, butnot exclusively, the invention relates to a transformer construction foruse in a power conditioning unit (inverter) such as those employed inphotovoltaic (PV) modules for delivering ac power either directly to themains (grid) utility supply or for powering mains (grid) devicesdirectly, independently from the mains utility supply.

BACKGROUND TO THE INVENTION

Transformers transfer electrical energy from one circuit to another viainductively coupled conductor coils. More specifically, a varyingcurrent in a primary coil winding creates a varying magnetic flux in acore of the transformer, which in turn generates a varying magneticfield (i.e. induced voltage) through a secondary coil winding. When aload is connected to the secondary winding, an electric current willflow in the secondary winding thereby transferring electrical energythrough the transformer to the load. The induced voltage in thesecondary winding is proportional to the primary voltage and isdetermined by the ratio of the number of turns in the secondary windingto the number of turns in the primary winding. A transformer maytherefore be configured to “step up” an ac voltage by making the numberof turns in the secondary winding greater than the number of turns inthe primary winding, or “step down” an ac voltage by making the numberof turns in the secondary winding less than the number of turns in theprimary winding.

Particular embodiments of the invention relate to transformers suitablefor use in power conditioning units for delivering power from a dc powersource (e.g. a photovoltaic module) to an ac output. Such powerconditioning units are described in the Applicant's earlier publishedpatent applications, for example, WO2007/080429. In that case, the powerconditioning unit comprises an input for receiving power from a dc powersource, an output for delivering ac power, an energy storage capacitor,a dc-to-dc converter having an input connection coupled to the input andan output connection coupled to the energy storage capacitor, and adc-to-ac converter having an input connection coupled to the energystorage capacitor and an output connection coupled to the output andwherein a transformer is provided in the dc-to-dc converter.

It is an aim of the present invention to provide a transformerconstruction with minimal size and optimum efficiency and which may besuitable for use in power conditioning units such as those describedabove.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided atransformer construction comprising: a plurality of transformer coresconfigured to share magnetic flux paths and wherein at least one of thecores comprises a post and an associated sidewall having an effectivecross-sectional area which is less than that of said post.

Embodiments of the present invention therefore provide transformerconstructions which employ shared magnetic paths thereby allowing theeffective cross-sectional area of a core sidewall to be reduced so as tominimise the overall volume of the transformer. This is important tomaximise the efficiency of the transformer since the volume is directlyproportional to core losses.

It is noted that in a traditional transformer core, the cross-sectionalarea of a centre post would match (i.e. balance) the effectivecross-sectional area of the associated sidewalls. For example, if thecross-sectional area of the post is 100 mm² and two sidewalls (e.g.limbs) are provided to close the magnetic path, each of the sidewallswould have a cross-sectional area of 50 mm²—giving a combined effectivecross-sectional area of 100 mm². The post would also be provided betweentwo end walls, each having a cross-sectional area of 50 mm². In theevent that three side walls were provided, each sidewall would require across-sectional area of 100 mm²/3 in order to balance the transformer.

Unless stated otherwise, the cross-sectional areas referred to throughthis specification will be taken to be transverse to the length of thestructures concerned.

Each post may be provided between two sidewall portions which combine toform said associated sidewall having a combined effectivecross-sectional area which is less than that of said post.

The posts and sidewalls may be mounted on a common substrate whereinadjacent posts share a common sidewall. In other words, the cores may beintegrated into a one-piece component. In certain embodiments, thisstructure may only form a first half of a transformer structure, and soa second half may be provided comprising an identical structure providedon a second substrate. In use, the first half and the second half willbe disposed adjacent each other such that the respective posts andsidewalls abut each other to form a symmetrical structure.

Alternatively, the transformer cores may be mounted on one or moreseparate substrates which are disposed adjacent each other to allowmagnetic flux to flow therebetween. As above, this structure may onlyform a first half of a transformer structure, and so a second half maybe provided comprising an identical structure provided on additionalsubstrates which are disposed facing each other to form a symmetricalstructure.

Each post may be mounted on a separate substrate having at least onesidewall provided thereon.

A first post may be mounted on a first substrate; a second post may bemounted on a second substrate; and a third post may be provided, part ofwhich is mounted on the first substrate and part of which is mounted onthe second substrate. In certain embodiments, this structure may onlyform a first half of a transformer structure, and so a second half maybe provided comprising an additional first post mounted on an additionalfirst substrate; an additional second post mounted on an additionalsecond substrate; and an additional third post provided, part of whichis mounted on the additional first substrate and part of which ismounted on the additional second substrate. It will be understood thatthe first half and the second half will be disposed adjacent each othersuch that the first post, second post and third post abut the respectiveadditional first post, additional second post and additional third post.

Three cores may be provided such that a middle post has two sidewallswhich are shared respectively by the posts on either side of the middlepost. Each shared sidewall may have a cross-sectional area which isgreater than a cross-sectional area of a sidewall which is not shared.Each shared sidewall may have a cross-sectional area which is twice thecross-sectional area of a sidewall which is not shared.

Each transformer post may have an oval cross-sectional area.

The associated sidewall may have an effective cross-sectional area whichis 10% to 40% less than that of said post. In a particular embodiment,the effective cross-sectional area of the associated sidewall may be 20%less than that of said post.

In accordance with a second aspect of the invention there is provided atransformer winding comprising a longitudinal spine having a first turnemanating from a first portion of the spine in a first transversedirection and a second turn emanating from a second portion of the spinein a second transverse direction; the second transverse direction beingopposite to the first transverse direction.

Embodiments of the second aspect of the invention therefore provide asimple and effective structure for a transformer winding, which can bequickly and easily manufactured.

The transformer winding may be formed from a planar metal sheet and thefirst and/or second turn may comprise one or more slits along itslength.

In accordance with a third aspect of the invention there is provided amethod of forming a transformer winding comprising:

-   -   providing a planar metal sheet;    -   cutting said sheet to form a longitudinal spine having a first        turn emanating from a first portion of the spine in a first        transverse direction and a second turn emanating from a second        portion of the spine in a second transverse direction, the        second transverse direction being opposite to the first        transverse direction; and    -   winding the first turn in a first direction and the second turn        in an opposite second direction.

The step of cutting said sheet may comprise stamping the required cutsin the sheet.

The method may further comprise forming one or more slits along thelength of the first and/or second turn.

In accordance with a fourth aspect of the invention there is provided atransformer bobbin structure comprising:

-   -   a primary bobbin mounted on a primary substrate;    -   a secondary bobbin mounted on a secondary substrate;    -   wherein the secondary bobbin is hollow and is configured for        receipt of at least a portion of a transformer core therein and        the primary bobbin is hollow and is configured for receipt of        the secondary bobbin therein; and    -   wherein at least one of the primary or secondary bobbins has a        free end comprising a flange which is configured to maintain a        pre-determined spacing between the primary and secondary        bobbins.

Embodiments of the fourth aspect of the invention therefore provide atransformer bobbin structure which can simply and effectively provide afixed physical separation between the primary and secondary bobbins suchthat leakage inductance will be controlled, which is a highly desirableeffect in resonant converters.

The primary or secondary bobbin may be provided between two sidewallsmounted on the respective primary or secondary substrate and which areconfigured to abut the other of the primary or secondary substrate whenthe secondary bobbin is received within the primary bobbin.

The transformer bobbin structure may further comprise a primary windingprovided around the primary bobbin and a secondary winding providedaround the secondary bobbin. The primary winding and/or the secondarywinding may be constituted by a transformer winding according to thesecond aspect of the present invention.

In accordance with a fifth aspect of the invention there is provided atransformer comprising the transformer construction according to thefirst aspect of the invention and/or a transformer winding according tothe second aspect of the invention and/or a transformer bobbin structureaccording to the fourth aspect of the invention.

In accordance with a sixth aspect of the invention there is provided apower conditioning unit comprising a transformer according to the fifthaspect of the invention and a drive circuit arranged to operate thetransformer.

In accordance with a seventh aspect of the invention there is provided apower conditioning unit for providing ac power from a photovoltaic (PV)panel comprising:

-   -   a plurality of single phase transformers; and    -   a plurality of drive circuits arranged to operate the plurality        of transformers out of phase from each other;    -   wherein the transformers are configured to share magnetic flux        paths.

Embodiments of the seventh aspect of the invention therefore provide apower conditioning unit in which the volume of each transformer can bereduced as a result of the transformers being configured to sharemagnetic flux paths, which is possible due to their operation beingout-of phase with each other.

Three transformers may be arranged to be operated by respective drivecircuits with a 60 degree phase shift therebetween.

In accordance with an eighth aspect of the invention there is provided amethod of operating a power conditioning unit for providing ac powerfrom a photovoltaic (PV) panel comprising:

-   -   providing a plurality of single phase transformers configured to        share magnetic flux paths, the transformers being arranged to be        operated by a plurality of associated drive circuits; and    -   operating the plurality of transformers out of phase from each        other.

The transformers referred to in the seventh and eighth aspects of theinvention may be in accordance with the fifth aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described,by way of example only, with reference to the accompanying figures, inwhich:

FIG. 1 shows an outline block diagram of an example power conditioningunit;

FIGS. 2 a and 2 b show details of a power conditioning unit of the typeshown in FIG. 1;

FIGS. 3 a and 3 b show details of a further example of solarphotovoltaic inverter;

FIGS. 4 a, 4 b and 4 c show transformer constructions according toembodiments of the present invention;

FIGS. 5 a, 5 b, 5 c and 5 d show a transformer bobbin structureaccording to embodiments of the present invention;

FIG. 6 shows the transformer construction of FIG. 4 c combined with thebobbin structure of FIG. 5 d according to an embodiment of the presentinvention;

FIG. 7 shows the transformer construction of FIG. 4 a combined with thebobbin structure of FIG. 5 d according to an embodiment of the presentinvention; and

FIGS. 8 a through 8 e show a transformer winding in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Power Conditioning Units

By way of background, we first describe an example photovoltaic powerconditioning unit. Thus FIG. 1 shows a photovoltaic power conditioningunit of the type we described in WO2007/080429. The power converter 1 ismade of three major elements: a power converter stage A, 3, a reservoir(dc link) capacitor C_(dc) 4, and a power converter stage B, 5. Theapparatus has an input connected to a direct current (dc) power source2, such as a solar or photovoltaic panel array (which may comprise oneor more dc sources connected in series and/or in parallel). Theapparatus also has an output to the grid main electricity supply 6 sothat the energy extracted from the dc source is transferred into thesupply. Capacitor C_(dc) is preferably non-electrolytic, for example afilm capacitor.

The power converter stage A may be, for example, a step-down converter,a step-up converter, or it may both amplify and attenuate the inputvoltage. In addition, it generally provides electrical isolation bymeans of a transformer or a coupled inductor. In general the electricalconditioning of the input voltage should be such that the voltage acrossthe dc link capacitor C_(dc) is always higher than the grid voltage. Ingeneral this block contains one or more transistors, inductors, andcapacitors. The transistor(s) may be driven by a pulse width modulation(PWM) generator. The PWM signal(s) have variable duty cycle, that is,the ON time is variable with respect to the period of the signal. Thisvariation of the duty cycle effectively controls the amount of powertransferred across the power converter stage A.

The power converter stage B injects current into the electricity supplyand the topology of this stage generally utilises some means to controlthe current flowing from the capacitor C_(dc) into the mains. Thecircuit topology may be either a voltage source inverter or a currentsource inverter.

FIGS. 2 a and 2 b show details of an example power conditioning unit ofthe type shown in FIG. 1; like elements are indicated by like referencenumerals. In FIG. 2 a Q1-Q4, D1-D4 and the transformer form a dc-to-dcconversion stage, here a voltage amplifier. In alternative arrangementsonly two transistors may be used; and/or a centre-tapped transformerwith two back-to-back diodes may be used as the bridge circuit.

In the dc-to-ac converter stage, Q9, D5, D6 and L_(out) perform currentshaping. In alternative arrangements this function may be located in aconnection between the bridge circuit and the dc link capacitor: D₆ actsas a free-wheeling diode and D₅ prevents current form flowing back intothe dc-link. When transistor Q₉ is switched on, a current builds upthrough L_(out). When Q₉ is switched off, this current cannot return tozero immediately so D₆ provides an alternative path for current to flowfrom the negative supply rail (D₅ prevents a current flowing back intothe dc-link via the body diode in Q₉ when Q₉ is switched off). Currentinjection into the grid is controlled using Q₉: when Q₉ is turned on thecurrent flowing through L_(out) increases and decreases when it isturned off (as long as the dc-link voltage is maintained higher than thegrid voltage magnitude). Hence the current is forced to follow arectified sinusoid which is in turn unfolded by the full-bridge output(transistors Q₅ to Q₈). Information from an output current sensor isused to feedback the instantaneous current value to a control circuit:The inductor current, i_(out,) is compared to a reference current,i_(ref), to determine whether or not to switch on transistor Q₉. If thereference current is higher than i_(out) then the transistor is turnedon; it is switched off otherwise. The reference current, i_(ref), may begenerated from a rectified sinusoidal template in synchronism with theac mains (grid) voltage.

Transistors Q5-Q8 constitutes an “unfolding” stage. Thus thesetransistors Q5-Q8 form a full-bridge that switches at line frequencyusing an analogue circuit synchronised with the grid voltage.Transistors Q5 and Q8 are on during the positive half cycle of the gridvoltage and Q6 and Q7 are on during the negative half cycle of the gridvoltage.

Thus in embodiments the power conditioning unit comprises a genericdc-ac-dc that provides voltage amplification of the source to above thegrid voltage, and isolation, and a current source inverter (CSI)connected to the mains. The current injection is regulated using currentshaping (current-control) in the inductor of the CSI via theintermediate buck-type stage. (This is described further in ourGB2415841B, incorporated by reference).

Control (block) A of FIG. 1 may be connected to the control connections(e.g. gates or bases) of transistors in power converter stage A tocontrol the transfer of power from the dc energy source. The input ofthis stage is connected to the dc energy source and the output of thisstage is connected to the dc link capacitor. This capacitor storesenergy from the dc energy source for delivery to the mains supply.Control (block) A may be configured to draw such that the unit drawssubstantially constant power from the dc energy source regardless of thedc link voltage V_(dc) on C_(dc).

Control (block) B may be connected to the control connections oftransistors in the power converter stage B to control the transfer ofpower to the mains supply. The input of this stage is connected to thedc link capacitor and the output of this stage is connected to the mainssupply. Control B may be configured to inject a substantially sinusoidalcurrent into the mains supply regardless of the dc link voltage V_(dc)on C_(dc).

The capacitor C_(dc) acts as an energy buffer from the input to theoutput. Energy is supplied into the capacitor via the power stage A atthe same time that energy is extracted from the capacitor via the powerstage B. The system provides a control method that balances the averageenergy transfer and allows a voltage fluctuation, resulting from theinjection of ac power into the mains, superimposed onto the average dcvoltage of the capacitor C_(dc). The frequency of the oscillation can beeither 100 Hz or 120 Hz depending on the line voltage frequency (50 Hzor 60 Hz respectively).

Two control blocks control the system: control block A controls thepower stage A, and control block B power stage B. An exampleimplementation of control blocks A and B is shown in FIG. 2 b. In thisexample these blocks operate independently but share a commonmicrocontroller for simplicity.

In broad terms, control block A senses the dc input voltage (and/orcurrent) and provides a PWM waveform to control the transistors of powerstage A to control the power transferred across this power stage.Control block B senses the output current (and voltage) and controls thetransistors of power stage B to control the power transferred to themains. Many different control strategies are possible. For exampledetails of one preferred strategy reference may be made to our earlierfiled WO2007/080429 (which senses the (ripple) voltage on the dclink)—but the embodiments of the invention we describe later do not relyon use of any particular control strategy.

In a photovoltaic power conditioning unit the microcontroller of FIG. 2b will generally implement an algorithm for some form of maximum powerpoint tracking. In embodiments of the invention we describe later thisor a similar microcontroller may be further configured to controlwhether one or both of the dc-to-dc power converter stages areoperational, and to implement “soft” switching off of one of thesestages when required. The microcontroller and/or associated hardware mayalso be configured to interleave the power transistor switching,preferably to reduce ripple as previously mentioned.

Now referring to FIG. 3 a, this shows a further example of a powerconditioning unit 600. In the architecture of FIG. 3 a a photovoltaicmodule 602 provides a dc power source for dc-to-dc power conversionstage 604, in this example each comprising an LLC resonant converter.Thus power conversion stage 604 comprises a dc-to-ac (switching)converter stage 606 to convert dc from module 602 to ac for atransformer 608. The secondary side of transformer 608 is coupled to arectifying circuit 610, which in turn provides a dc output to aseries-coupled output inductor 612. Output inductor 612 is coupled to adc link 614 of the power conditioning unit, to which is also coupled adc link capacitor 616. A dc-to-ac converter 618 has a dc input from a dclink and provides an ac output 620, for example to an ac grid mainssupply.

A microcontroller 622 provides switching control signals to dc-to-acconverter 606, to rectifying circuit 610 (for synchronous rectifiers),and to dc-to-ac converter 618 in the output ‘unfolding’ stage. Asillustrated microcontroller 622 also senses the output voltage/currentto the grid, the input voltage/current from the PV module 602, and, inembodiments, the dc link voltage. (The skilled person will be aware ofmany ways in which such sensing may be performed). In some embodimentsthe microcontroller 622 implements a control strategy as previouslydescribed. As illustrated, the microcontroller 622 is coupled to an RFtransceiver 624 such as a ZigBee™ transceiver, which is provided with anantenna 626 for monitoring and control of the power conditioning unit600.

Referring now to FIG. 3 b, this shows details of a portion of an exampleimplementation of the arrangement of FIG. 3 a. This example arrangementemploys a modification of the circuit of FIG. 2 a and like elements tothose of FIG. 2 a are indicated by like reference numerals; likewiselike elements to those of FIG. 3 a are indicated by like referencenumerals. In the arrangement of FIG. 3 b an LLC converter is employed(by contrast with FIG. 2 a), using a pair of resonant capacitors C1, C3.

The circuits of FIGS. 1 to 3 are particularly useful for microinverters,for example having a maximum rate of power of less than 1000 Watts andor connected to a small number of PV modules, for example just one ortwo such modules. In such systems the panel voltages can be as low as 20volts and hence the conversion currents can be in excess of 30 amps RMS.

Transformer Constructions

We will now describe transformer constructions which may be employed inthe circuits of FIGS. 1 to 3.

FIG. 4 a shows a transformer construction 100 in accordance with anembodiment of the present invention. The transformer construction 100comprises three transformer cores 102 which are conjoined and aretherefore configured to share magnetic flux paths. Each core 102comprises a post 104 provided between two sidewall portions 106 whichcombine to form an associated sidewall having a combined effectivecross-sectional area which is 20% less than that of the post 104. Morespecifically, the posts 104 each have a cross-sectional area of 100 mm²and the associated sidewall has an effective cross-sectional area of 80mm². The posts 104 and sidewalls 106 are mounted on a common substrate(i.e. back wall) 108 and are arranged such that adjacent posts 104 sharea common sidewall 106.

It will be noted that the three cores 102 are provided such that amiddle post 104 has two sidewalls 106 which are shared respectively bythe posts 104 on either side of the middle post 104. Each sharedsidewall 106 also has a cross-sectional area which is greater than across-sectional area of a sidewall 106 which is not shared. In thisparticular embodiment, each post 104 has an oval cross-sectional area asthis further helps to minimise the volume of the transformer whilstallowing for optimum efficiency.

The transformer construction 100 may be employed in a power conditioningunit similar to that described above but wherein three separate DCsources are provided (e.g. constituted by three photovoltaicsubstrings). In which case, three separate drive circuits may beemployed to operate the three cores 102 60 degrees out of phase fromeach other on a primary side of the transformer. Three outputs from thesecondary side of the transformer may be connected in parallel beforefeeding into a dc link capacitor as described previously. The fact thatthe cores 102 are operated in a particular sequence (i.e. phasing)allows the cores 102 to share magnetic flux paths without creatinginterference. Accordingly, the transformer core 102 volumes can beminimised without adversely affecting performance. In fact, inparticular embodiments of the invention it is possible to minimise thesize and optimise the efficiency of the transformer since the volume ofthe transformer core is directly proportional to the core losses.

FIG. 4 b shows a variant of the transformer construction 100 and so likeparts will be indicated using like reference numerals. In essence, FIG.4 b shows a single transformer core 110 which may be magneticallycoupled (e.g. connected) to two similar cores 110 to form the samestructure as shown in FIG. 4 a although this time it is formed fromthree discrete cores 110. Thus, the transformer core 110 comprises apost 104 provided between two sidewall portions 106. When two or morecores 110 are magnetically coupled together their respective sidewallportions 106 will combine to form an associated sidewall having acombined effective cross-sectional area which is 20% less than that ofthe post 104. The post 104 and two sidewalls 106 are mounted on anindividual substrate (i.e. back wall) 112. As shown in FIG. 4 b, eachsidewall 106 is of the same thickness such that when two cores 110 arecoupled together the effective thickness of the shared sidewall isdoubled.

A further embodiment of the present invention is shown in FIG. 4 c. Thisconstruction 120 effectively forms half of the transformer construction100 of FIG. 4 a and so like reference numerals will be employed for likeparts. The transformer construction 120 therefore comprises a first post104 and a first half of a second post 121 mounted on a first substrate122. Two sidewalls 106 are provided around the first post 104 such thatone of these forms a sidewall 106 that is shared with the first half ofthe second post 121. As described previously, the shared sidewall 106 istwice the thickness of the sidewall 106 that is not shared. It will beunderstood that in practice a second half of the construction 120 willbe provided which will form a mirror image of the construction shown andwhich will comprise a second half of the second post 121 as well as athird post with associated sidewalls. Thus, the first and second halveswill be configured such that they can be coupled together to formessentially the same structure as illustrated in FIG. 4 a.

FIG. 5 a shows a secondary bobbin structure 130 comprising an ovalhollow (secondary) support 132—configured for receipt of a post 104 ofone of the transformer cores described—mounted in the centre of arectangular planar secondary substrate 134 which has a central holetherethrough. A free end of the support 132 comprises an outwardlyextending radial flange 136. At a lower end of the flange 136 there isprovided a cuboidal feeder 138 having two discrete longitudinal channels140 therethrough. As shown in FIG. 5 b, a secondary winding 142 ofcylindrical wire is wound around the outer surface of the support 132and the two free ends 144 of the winding 142 are passed through the twolongitudinal channels 140. Although not shown, these ends 144 will beconnected to a secondary circuit to drive the transformer such asprovided in the power conditioning units discussed above.

As shown in FIG. 5 c, a primary bobbin structure 150 is provided whichcomprises a an oval hollow (primary) support 152 mounted in the centreof a rectangular planar primary substrate 154 which has a central hole156 therethrough. Two sidewalls 158 are provided on the primarysubstrate 154 on either side of the support 152. As illustrated, theprimary support 152 is placed around the secondary support 132 until thefree ends of the primary support 152 and sidewalls 158 abut thesecondary substrate 134. At the same time, the flange 136 will abut theprimary substrate 154 and the inside of the primary support 152 tothereby maintain a desired spacing between the primary and second bobbinstructures.

As shown in FIG. 5 d, a primary winding 160 is wound around the primarysupport 152. Note, the primary winding 160 is likely to be provided onthe primary support 152 before the primary bobbin 150 is placed over thesecondary bobbin 130.

The two-piece transformer bobbin structure 130, 150 shown in FIGS. 5 ato 5 d can provide improved manufacturability and efficiency performancein a transformer, particularly when combined with a transformer coreconstruction of FIGS. 4 a to 4 c. More specifically, the bobbinstructure can achieve tight tolerances on leakage inductance by ensuringa fixed physical separation between the primary and secondary windings160, 142.

FIG. 6 shows the transformer construction of FIG. 4 c combined with thebobbin structure of FIG. 5 d according to an embodiment of the presentinvention. For ease of illustration, three out of four of thetransformer core constructions 120 required in this embodiment areshown. More specifically, two constructions 120 are required to form atwo-piece structure similar to that of FIG. 4 a and the same structureagain is required to double the length of each of the posts 104 and toform a complete and enclosed transformer structure. The transformerbobbin structure 130, 150 of FIG. 5 d is provided around each of thethree (doubled) posts 104 (although for clarity, the primary bobbinstructure 150 is not shown around the last post 104). Thus, thecombination of the transformer construction and the bobbin structureprovides a compact and efficient transformer which can be employed in apower conditioning unit as described above.

FIG. 7 is similar to FIG. 6 but this time shows the transformerconstruction of FIG. 4 a combined with the bobbin structure of FIG. 5 daccording to an embodiment of the present invention. Accordingly, onlytwo of the transformer constructions 100 are required along with threeof the transformer bobbin structures 130, 150, which are positioned overeach post 104.

FIGS. 8 a through 8 e show a transformer winding 180 in accordance withan embodiment of the present invention. The transformer winding 180 mayform the primary winding 160 described above. As shown in FIG. 8 a, thetransformer winding 180 is stamped from a planar copper sheet andcomprises a longitudinal spine 182 having a first turn 184 emanatingfrom a first portion 186 of the spine 182 in a first transversedirection and a second turn 188 emanating from a second portion 190 ofthe spine 182 in a second transverse direction which is opposite to thefirst transverse direction. Each turn 184, 188 comprises a slit 194along the centre of its length which helps to improve efficiencyminimizing eddy current loss.

In order to make the planar sheet of FIG. 8 a into the winding 182illustrated in FIGS. 8 b to 8 e, the first turn 184 is wound in a firstdirection (i.e. in a clockwise direction around a bobbin or core) andthe second turn 188 is wound in an opposite (i.e. anti-clockwise) seconddirection. Accordingly, the winding 180 has a simple 2 turn structurewhich is quick and easy to manufacture and assemble.

Advantages of Various Embodiments

Some of advantages of the transformer structures and related featuresdescribed above are as follows:

-   -   1) A high efficiency integrated magnetic structure that shares        magnetic flux paths for volume optimisation;    -   2) Lower volume structure compared to single discrete        transformers;    -   3) Stamped 2 turn primary winding with a simple (one forward,        one back) fold forming sequence to achieve low dc resistance and        low ac winding structure;    -   4) A 2 piece bobbin structure optimised for tight leakage        inductance tolerance for improved resonant operation        performance.

Accordingly, the various aspects of the present invention can provide atransformer construction or parts therefore with improvedmanufacturability and efficiency. A multiple piece manufacturingapproach can be employed, that can be automated, such that allcomponents of the core, windings and bobbins can be manufacturedoff-line and integrated together in a simple (few step) manufacturingprocess. Consequently, manufacturing errors can be minimised andreliability can be increased.

When employed in a power conditioning unit, the transformer is a keycomponent in relation to overall efficiency and so each of the featuresand techniques described above which help minimise the losses associatedwith the transformer will greatly improve the overall efficiency of thepower conditioning unit.

No doubt many other effective alternatives will occur to the skilledperson. It will be understood that the invention is not limited to thedescribed embodiments and encompasses modifications apparent to thoseskilled in the art which lie within the spirit and scope of the appendedclaims.

We claim:
 1. A power conditioning unit for providing ac power from aphotovoltaic (PV) panel comprising: a plurality of single phasetransformers; and a plurality of drive circuits arranged to operate theplurality of transformers out of phase from each other; wherein thetransformers are configured to share magnetic flux paths.
 2. The powerconditioning unit according to claim 1 wherein three transformers arearranged to be operated by respective drive circuits with a 60 degreephase shift therebetween.
 3. The power conditioning unit of claim 1,wherein each transformer comprises a primary side and a secondary side,wherein each driver circuits operates a transformer from the primaryside of the transformer.
 4. The power conditioning unit of claim 3,wherein the plurality of transformers are connected in parallel at theirsecondary side before feeding into a dc link capacitor that storesenergy from the PV panel.
 5. The power conditioning unit of claim 1,wherein the plurality of transformers comprises a plurality cores thatare conjoined to share magnetic flux paths.
 6. The power conditioningunit of claim 5, wherein the plurality of the transformers are operatedin a particular sequence to allow the cores to share magnetic flux pathswithout creating interference.
 7. The power conditioning unit of claim5, wherein at least one of the cores comprises a post and an associatedsidewall having an effective cross-sectional area which is less thanthat of said post.
 8. The power conditioning unit of claim 7, whereinthe posts and sidewalls are mounted on a common substrate, whereinadjacent posts share a common sidewall, wherein the transformer coresare mounted on one or more separate substrates which are disposedadjacent to each other to allow magnetic flux to flow between thetransformer cores.
 9. A method of operating a power conditioning unitfor providing ac power from a photovoltaic (PV) panel comprising:providing a plurality of single phase transformers configured to sharemagnetic flux paths; and providing a plurality of drive circuits foroperating the plurality of transformers out of phase from each other.10. The method of claim 9, wherein three transformers are operating at60 degree phase shift therebetween.
 11. The method of claim 9, whereineach transformer comprises a primary side and a secondary side, whereineach driver circuits operates a transformer from the primary side of thetransformer.
 12. The method of claim 11, wherein the plurality oftransformers are connected in parallel at their secondary side beforefeeding into a dc link capacitor that stores energy from the PV panel.13. The method of claim 9, wherein the plurality of transformerscomprise a plurality of cores that are conjoined to share magnetic fluxpaths.
 14. The method of claim 13, wherein the plurality of transformersare operated in a particular sequence to allow the cores to sharemagnetic flux paths without creating interference.
 15. The method ofclaim 13, wherein at least one of the cores comprises a post and anassociated sidewall having an effective cross-sectional area which isless than that of said post.
 16. The method of claim 15, wherein theposts and sidewalls are mounted on a common substrate, wherein adjacentposts share a common sidewall, wherein the transformer cores are mountedon one or more separate substrates which are disposed adjacent to eachother to allow magnetic flux to flow between the transformer cores. 17.A method for providing ac power from a photovoltaic (PV) panel, themethod comprising: converting dc power from the PV panel in a pluralityof single phase transformers having conjoined cores that share magneticflux paths; configuring a plurality of drive circuits for operating theplurality of single phase transformers to be at different phases toallow the cores of the transformers to share the magnetic flux pathswithout interference.
 18. The method of claim 17, wherein the pluralityof the transformers are operated in a particular sequence to allow thecores to share magnetic flux paths without creating interference. 19.The method of claim 17, wherein three transformers are operating at 60degree phase shift therebetween.